The present invention relates to an improved convergence circuit for correcting a misconvergence which takes place in a multielectron beam cathode-ray tube.
As well known, a color cathode-ray tube in which three electron beams are emitted in an in-line arrangement is widely employed in various electrical appliances such as television receiver sets, video monitors, CRT displays, etc.
A cathode-ray tube is provided with a deflection yoke which deflects three electron beams to the internal fluorescent surface of the cathode-ray tube. In this case, a misconvergence occurs in rasters on the fluorescent surface for a reason attribute to the construction of the cathode-ray tube such as, for example, the curvature of the fluorescent surface, inclination of the electron gun and error of the assembly in manufacture or for a reason attribute to the construction of the deflection yoke such as, for example, a failure in obtaining a desired shape and conductor distribution of the deflection coil. Correction of this kind of misconvergence assumes the use of an independent convergence yoke as described in the U.S. Pat. No. 3,912,970. In case of the in-line type cathode-ray tube, the deflection yoke is required to correct by itself the misconvergence for an economical reason. However, it is difficult to design the deflection yoke in a construction in which no misconvergence takes place. Therefore, the deflection yoke is designed so that a misconvergence of three rasters on the fluorescent surface which are approximately aligned in the vertical direction and parabolic only in horizontal direction remains in the vertical direction Y of screen S as shown in FIG. 1 and a cross misconvergence which takes place at the upper and lower end parts of the screen and which is difficult to be corrected does not remain. In other words, the misconvergence which inevitably takes palce for the design of the deflection yoke is caused with intensive misconvergence value Y.sub.H of the horizontal component from which the component in the Y direction is eliminated.
Such misconvergence can be easily corrected by the correction coil devices 10 and 11 as shown in FIG. 2. These devices 10 and 11 are arranged opposing each other in the direction of in-line arrangement of three electron beams B, G and R at the outside of the cathode-ray tube neck 17. Correction coil devices 10 and 11 comprise U-shaped magnetic cores 10a and 11a and correction coils 10b and 11b which are wound around said magnetic cores. Correction coils 10b and 11b are series-connected to the convergence current source 12 to generate magnetic fluxes 13, 14, 15 and 16 in the arrowhead direction inside the neck 17. Such correction coil devices are described in detail in the U.S. Pat. No. 4,090,110.
Value Y.sub.H of misconvergence which will appear on the screen in the manufacturing processes for television receiver sets, etc. differs with each assembly of the cathode-ray tube and the deflection yoke which are assembled together in the manufacturing processes. Therefore, a amplitude adjusting means which permits correction in response to the magnitude of misconvergence value Y.sub.H is demanded. As shown in FIG. 3, the in-line arrangement of three electron beams is displaced in reference to the X axis of the screen. In this case, the misconvergence value Y.sub.H1 at the upper end part of the screen differs from the misconvergence value Y.sub.H2 at the lower end part of the screen and accordingly such misconvergence cannot be corrected only by said amplitude adjusting means. Therefore, a tilt adjusting means which adjusts electron beams B, G and R to correct positions.